This invention relates generally to radio frequency (RF) integrated circuits that include active devices and passive components such as inductors and capacitors used for radio frequency devices such as cellular telephones and wireless network devices such as Bluetooth and other wireless devices and personal digital assistants.
The technology for manufacturing integrated circuits has conventionally divided integrated circuits into various categories based on the compatibility of processes and other considerations. Generally, radio frequency circuits have not been mixed with logic circuits in the same integrated circuit. Radio frequency circuits are analog circuits that are involved in filtering and detecting radio frequency signals such as cellular telephone signals. In contrast, logic circuits generally include transistors and other active components that form digital integrated circuit devices. Thus, for example, bipolar techniques may be utilized to manufacture radio frequency circuits and standard complementary metal oxide semiconductor (CMOS) processes may be utilized to manufacture logic circuits.
Memory circuits may account for still another category. Generally, special processes may be utilized in connection with the manufacture of memory circuits because of special design considerations such as multiple gate electrodes and special voltage supply needs. Thus, memory circuits are often fabricated separately from logic circuits.
Still another category are the so-called mixed signal circuits which may include both digital and analog components. These signals too may be accounted for separately so that a device that includes RF signal processing, RF integrated circuits, mixed signal circuits, logic circuits and memory circuits may be made up of a number of separately fabricated integrated circuit chips.
The cost of an electronic device may be closely correlated to the extent of integration that is possible. The more devices and the more types of devices that can be integrated into a single integrated circuit and manufactured using highly replicated techniques, the lower the resulting price. Unfortunately, because of incompatibilities between the different types of integrated circuits, it has not been possible, to date, to fabricate both radio frequency circuits, mixed signal circuits, logic circuits and memory circuits all on the same standard CMOS integrated circuit process.
One problem that arises in connection with radio frequency circuits in CMOS processes is that the passive components such as capacitors and inductors may be adversely affected by the substrates over which they are formed. In particular, coupling may occur between the substrate and integrated circuit inductors for example. This coupling may result in degraded performance of inductive circuits. As a result, inductive circuits may be formed in bipolar or silicon over insulator (SOI) integrated circuits rather than using standard CMOS logic processes. Thus, two or more integrated circuits are needed--one for logic, one for RF circuits, one for memory and one for mixed signals.
Some efforts have been made to overcome this coupling problem. For example, Silicon Wave Inc. has devised a so-called silicon over insulator (SOI) BiCMOS (Bipolar CMOS) integrated circuit which integrates both logic and radio frequency components onto the same die. However, the use of silicon over insulator technology greatly complicates the manufacturing process and increases cost. Moreover, the bulk of semiconductor fabrication facilities in the United States and the rest of the world are dedicated to manufacturing metal oxide semiconductor technologies. The SOI process is not amenable to widespread manufacturing at a number of highly expensive, already existent, fabrication facilities.
In addition to capacitive coupling of substrate noise, magnetic coupling between integrated inductors on the same integrated circuit may also be problematic. The coupling between integrated inductors on the same circuit may adversely effect the operation of any given inductor. For example, the magnetic coupling may change the operating parameters of one or more integrated inductors on the same integrated circuit.
The quality factor or Q factor strongly depends on the layout and the properties of the particular process technology. It is known that the quality of an integrated inductor may be adversely affected by the metal wire resistance, capacitive coupling to the substrate and magnetic coupling to the substrate. Capacitive and magnetic coupling limit the Q factor at relatively high frequencies. The magnetic coupling becomes more significant in CMOS technologies with doped substrates because the effect of substrate resistance appears in parallel with the inductor.
Thus, there is a substantial need to find a way to reduce coupling in integrated inductors.